Fix TypeError: args[1].toUpperCase is not a function

Issue #389 reported by Keyop:
srvc 0324 IMAPS <Keyop> !JavaScript  /sbbs/exec/imapservice.js line 653:
TypeError: args[1].toUpperCase is not a function

I'm not sure how you reproduce this error or what type args[1] is
(apparently, not a string), but this fix it regardless. Please let me
know.

exec/imapservice.js

@@ -649,7 +649,7 @@ function handle_command(command, args, defs)
 function parse_command(line)
 {
 	function execute_line(args) {
-		if(args.length >= 2) {
+		if(args.length >= 2 && typeof(args[1]) == 'string') {
 			command=args[1].toUpperCase();
 			args.splice(1,1);
 			if(handle_command(command, args, any_state_command_handlers))